The Anglo-French team made the discovery following a study of 3,673 British civil servants working in London, they were part of a larger study of over 10,000 people taking part in a long term health investigation, which commenced in 1985.

The scientists found that participants with low levels of high density lipoprotein (HDL) were more likely to suffer memory decline by the age of 60 and suggest that a lack of "good" cholesterol in middle age may increase the risk of memory loss and Alzheimer's.

They found that at the age of 55, participants with low HDL cholesterol had a 27% increased risk of memory loss compared with those with high HDL and five years later, low HDL members of the group had a 53% increased risk of memory loss.

The scientists from University College London and the INSERM institute in France, found that participants with low levels of HDL were more likely to suffer memory decline by the age of 60.

Unlike low density lipoprotein (LDL) or "bad" cholesterol, HDL protects against narrowing of the arteries and heart disease, as it helps to clear excess cholesterol from the blood, and assists the maturation of nerve cell synapses - the junction points where different neurons communicate which are important to memory.

Another effect of HDL is the regulation of the formation of amyloid-beta protein in the brain - deposits of amyloid-beta are one of the hallmarks of Alzheimer's disease.

The scientists defined low HDL as less than 40 milligrams per decilitre of blood (mg/dL), and high HDL as 60 mg/dL or more - blood samples were taken from the civil servants on two occasions five years apart and their short term verbal memory was assessed with a word-recall test.

Lead author Dr. Archarna Singh-Manoux says memory problems are key in the diagnosis of dementia and the research suggests that low HDL cholesterol might be a risk factor for dementia.

Levels of HDL can be boosted by regular exercise and eating less saturated fat and more "healthy" fats such as olive oils.

Dr. Singh-Manoux suggest that doctors should be encouraged to monitor HDL levels in order to predict dementia risk but experts say while blood tests might reveal high-risk patients, the evidence for larger diet trials aimed at boosting HDL levels are not justified.

The Australian Institute of Health and Welfare (AIHW) says figures show the burden of dementia in Australia will continue to grow and 465,000 Australians will be suffering from some form of dementia by 2031.

The research is published in the American Heart Association journal Arteriosclerosis, Thrombosis and Vascular Biology.